Shielded temporary plugging is a reservoir protection technique widely applied in well drilling and completion processes. The shielded temporary plugging technique is to add a temporary plugging agent into the drilling fluid to form a dense temporary plugging layer, so as to prevent reservoir damaged incurred by invasion of the solid phase and liquid phase in the drilling fluid into the reservoir. The temporary plugging agent can form a high-quality temporary plugging zone only if it perfectly matches with the pore throat diameter of the formation, but it is very difficult to realize perfect match in the actual production process. Consequently, a selected temporary plugging agent is only applicable to a specific well section, but attains a poor temporary plugging effect if it is used throughout the oil and gas formation. To mitigate that problem, adaptive shielded temporary plugging agents have been developed. Such adaptive shielded temporary plugging agents employ polymers as the primary component, and utilize deformable elastic particles and packing and strengthening agents as auxiliary agents. However, many new polymeric temporary plugging agents have poor biodegradability, which results in environmental protection problems.
Water-absorbent resins are a sort of new macromolecular functional materials. They are polymers that swell but are not dissolved in water, have three-dimensional mesh structures that are at certain degree of cross-linking, have deformability, and can be used to plug up pore canals in different sizes by means of shape adjustment. Presently, water-absorbent resins have been widely applied in oil development, but they have drawbacks such as poor degradability and high toxicity, etc.
Starch-based water-absorbent resins use starch as the main chain. Starch-grafted polymers that use vinyl polymers containing a large quantity of hydrophilic groups as side chains can be used as super absorbent resins, which have unique properties of swelling while absorbing water and degradability. Those properties bring a new clue for the technique of degradable starch-based water-absorbent resin temporary plugging agent for drilling fluid. Starch is a renewable resource, has active hydroxyl groups in starch molecules, and is easy to be modified chemically and physically. Modified starch-based materials have biodegradability. Besides, acid-dissoluble rigid particles may be adulterated in starch-based water-absorbent resins to form organic-inorganic hybrid grid structures and thereby improve the overall properties of the water-absorbent resins. Therefore, starch-based water-absorbent resins are a sort of potential environment-friendly temporary plugging materials for drilling fluids.
CN104449613A has disclosed a water-absorbent resin temporary plugging agent and preparation method thereof. The water-absorbent resin temporary plugging agent is prepared from an acrylic monomer, an acrylamide monomer, and an acid-dissoluble inorganic packing material in powder form. The water-absorbent resin may be used for plugging leaking channels in different shapes and different sizes, but is non-biodegradable.
CN104710967A has disclosed a self-removing temperature-resistant temporary plugging agent for water-based drilling fluid and preparation method thereof. The temporary plugging agent consists of a product obtained through reactions of starch, alkali hydroxide, halogen organic acid, carbonate, hydrophobic modifier, alcohol and water, and nanometer calcium carbonate, has high temperature-resistance property, and can attain a good temporary plugging effect for well cores different in permeability, but its constituents contain benzene rings, which are difficult to biodegrade.
CN106883832A has disclosed an environment-friendly controllable temporary plugging agent and preparation method thereof. The temporary plugging agent is applicable to shale gas extraction, utilizes environment-friendly raw materials, and has good biocompatibility, but don't have adaptive temporary plugging ability for well cores different in permeability.
In summary, existing temporary plugging agents don't have environment friendliness as well as adaptive temporary plugging ability, and most of them are used in profile modification, water plugging, or fracturing techniques. There is no report on any temporary plugging agent for drilling fluid that has environment friendliness as well as adaptive temporary plugging ability in China yet.
Therefore, it is of great significance to develop a temporary plugging agent that has environment friendliness as well as adaptive plugging ability for formations different in permeability.